1. Field of the Invention
This invention relates to a control system for an automatic vehicle transmission.
2. Description of the Related Art
A conventional automatic vehicle transmission has a console-type range selector which is installed in a console box provided at the floor close to the vehicle operator seat. The console type range selector is mechanically connected to a hydraulic valve (i.e., the manual valve) in a hydraulic mechanism in order that a desired one from among various gear ranges is selected by the vehicle operator through manipulation of a lever of the range selector. The range or position selected by the vehicle operator is detected by a position switch which generates and sends a signal indicative of the selected range to a control unit of the hydraulic mechanism where necessary control is conducted.
On the other hand, in order to enlarge free space around the vehicle operator seat, it has been proposed to provide, instead of the console-type range selector, a range selector switch (an electric switch) at or near the steering wheel, etc., as well as an electric motor for moving the manual valve or a parking mechanism in the so-called DBW (Drive By Wire) fashion. This is disclosed, for example, in Japanese Laid-Open Patent Application Nos. Hei 7 (1995)-253149, Hei 7 (1995)-253150, Hei 7 (1995)-253151, Hei 7 (1995)-253155, and Hei 7 (1995)-251645.
Since, however, these prior art systems are configured such that the parking mechanism is to be driven by an electric motor, the prior art systems require an electric motor of large power. This is disadvantageous in that it increases consumption of electric power supplied from the on-board battery and needs a large space to accommodate the electric motor.
Moreover, the prior art systems are disadvantageous in that they require the same number of sensors as that of ranges (positions) to detect the range (position) selected by the vehicle operator.
Furthermore, the prior art systems are disadvantageous in that, if the battery is completely discharged, it becomes impossible to release the parking mechanism to move the vehicle. The vehicle operator will encounter a similar problem when trying to park the vehicle with a discharged battery.